The present invention relates to electron gun structures, and more particularly an in line type electron gun structure capable of improving the focusing characteristics over all the screen of color picture tube.
Conventionally known color picture tubes of a shadow mask type were first introduced to the public in 1972 by RCA, USA. Since this color picture tube was a self-convergence type using an integral in line electron gun structure, it has radically rationalized color picture tubes and color television sets and today, has been most popular. However, in the color picture tube constructed as above, the focusing characteristics are quite degraded, because the three electron guns are integrally arranged in line without modifying the inner diameter of the neck used heretofore in delta configuration, that is, the neck inner diameter adapted to house three electron guns arranged in delta configuration, thereby making the diameter of the electrode group forming electron lens substantially smaller and automatically increasing the spherical aberration of the electron lens.
Further, self-convergence system forms a strong pin cushion distortion in the horizontal deflection magnetic field and a strong barrel distortion in the vertical deflection magnetic field. Thus, the electron beams which was originally round is subjected to the distortions of the deflection magnetic fields and greatly distorted in its spot shape at the periphery of the color picture tube screen, thereby deteriorating the focus on the screen.
In order to recover the focus property impaired by the spherical aberration, the potential difference between the electrodes in the electron beam constituting the main lens is decreased and the focus voltage is increased to decrease the spherical aberration so that the diameter of the electron beam in the main lens is increased. As a result, interacting repulsive force of electrons in the electron beam is decreased to thereby decrease the divergence of the electron beam, attaining a small spot diameter. Among other measures to improve the focus property, there is proposed an expedient wherein the main lens is divided into a plurality of sub-lenses to gradually focus the electron beam, thereby decreasing the spherical aberration substantially and increasing the electron beam diameter in the main lens and consequently, the divergence of electron beam to be focused on the fluorescent screen is decreased; such an expedient uses, for example, so-called multi-stage focusing type electron gun such as high bi-potential gun, high uni-potential gun, tri-potential gun for improving the focusing property discussed above. Although thus improved focusing property is particularly remarkable in the center of the fluorescent screen of the color picture tube, they become conversely inferior at the periphery of the screen. This is due to the effect of the self-convergence type deflection magnetic fields wherein the above discussed strong distortions are quite evident. In other words, due to the fact that the diameter of the electron beam is increased in the main lens, angle of the electron beam with respect to the deflection magnetic field is increased as the electron beam passes through the deflection magnetic field. For these reasons, the electron beam is accompanied by an oval core in the x-axis direction and a halo in the y-axis direction of color picture tube, for instance, at the corners of the color picture tube.